Squire's theorem
= Squire's theorem
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= Squire theorem
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For temporal normal-mode stability of a viscous parallel shear flow, any unstable three-dimensional mode maps by the <Squire transformation> to an unstable two-dimensional mode at a no larger <Reynolds number>. Hence the first temporal modal instability occurs among two-dimensional disturbances. The theorem does not exclude three-dimensional <transient growth> or forced responses.